1. Field of the Invention
This invention relates to welded steel pipes suitable for forming structural components and underbody components of vehicles. In particular, the invention relates to enhancement of hydroformability of welded steel pipes.
2. Description of the Related Art
Hollow structural components having various cross-sectional shapes are used in vehicles. Such hollow structural components are typically produced by spot welding parts formed by press working of a steel sheet. Since hollow structural components of current vehicles must have high shock absorbability for collision impact, the steels used as the raw material must have higher mechanical strength. Unfortunately, such high-strength steels exhibit poor press formability. Thus, it is difficult to produce structural components having highly precise shapes and sizes without defects from the high-strength steels by press molding.
A method that attempts to solve such a problem is hydroforming in which the interior of a steel pipe is filled with a high-pressure liquid to deform the steel pipe into a component having a desired shape. In this method, the cross-sectional size of the steel pipe is changed by a bulging process. A component having a complicated shape can be integrally formed and the formed component exhibits high mechanical strength and rigidity. Thus, the hydroforming attracts attention as an advanced forming process.
In the hydroforming process, electrically welded pipes composed of low or middle carbon content steel sheet containing 0.10 to 0.20 mass percent carbon are often used due to high mechanical strength and low cost. Unfortunately, electrically welded pipes composed of low or middle carbon content steel have poor hydroformability; hence, the pipes cannot be sufficiently expanded.
A countermeasure to enhance the hydroformability of electric welded pipes is the use of ultra-low carbon content steel sheet containing an extremely low amount of carbon. Electrically welded pipes composed of the ultra-low carbon content steel sheet exhibit excellent hydroformability. However, crystal grains grow to cause softening of the pipe at the seam during the pipe forming process, so that the seam is intensively deformed in the bulging process, thereby impairing the high ductility of the raw material. Thus, welded pipes must have excellent mechanical properties durable for hydroforming at the seam.
An object of the invention is to provide a welded steel pipe having excellent hydroformability durable for a severe hydroforming process.
Another object of the invention is to provide a method for making the welded steel pipe.
In the invention, the welded steel pipe has a tensile strength TS of at least about 400 MPa, preferably in the range of about 400 MPa to less than about 590 MPa, and a product nxc3x97r of the n-value and the r-value of at least about 0.22 and, preferably, an n-value of at least about 0.15 and an r-value of at least about 1.5.
We intensively investigated compositions of welded steel pipes and methods for making the welded steel pipes to solve the above problems and discovered that a welded steel pipe that contains about 0.05 to about 0.2 mass percent carbon and that is reduction-rolled at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500 to about 900xc2x0 C. has a high nxc3x97r product (product of an n-value and an r-value) and exhibits excellent hydroformability.
According to a first aspect of the invention, a welded steel pipe having excellent hydroformability has a composition comprising, on the basis of mass percent, about 0.05% to about 0.2% C; about 0.01% to about 0.2% Si; about 0.2% to about 1.5% Mn; about 0.01% to about 0.1% P; about 0.01% or less of S; about 0.01% to about 0.1% Al; about 0.001% to about 0.01% N; and the balance being Fe and incidental impurities, wherein the tensile strength of the welded steel pipe is at least about 400 MPa, preferably in the range of about 400 MPa to less than about 590 MPa, and the nxc3x97r product of the n-value and the r-value is at least about 0.22. Preferably, the n-value is at least about 0.15 or the r-value is at least about 1.5. Preferably, the composition further comprises at least one group of Group A and Group B, wherein Group A includes at least one element of about 0.1% or less of Cr, about 0.05% or less of Nb, about 0.05% or less of Ti, about 1.0% or less of Cu, about 1.0% or less of Ni, about 1.0% or less of Mo, and about 0.01% or less of B; and Group B includes at least one element of about 0.02% or less of Ca and about 0.02% or less of a rare earth metal.
According to a second aspect of the invention, a method for making a welded steel pipe having excellent hydroformability comprises: heating or soaking an untreated welded steel pipe having a steel composition containing, on the basis of mass percent: about 0.05% to about 0.2% C; about 0.2% or less of Si; about 1.5% or less of Mn; about 0.1% or less of P; about 0.01% or less of S; about 0.1% or less of Al; and about 0.01% or less of N; and reduction-rolling the treated steel pipe at a cumulative reduction rate of at least about 35% and a final rolling temperature of about 500xc2x0 C. to about 900xc2x0 C., the welded steel pipe thereby having a tensile strength of at least about 400 MPa and an nxc3x97r product of an n-value and an r-value of at least about 0.22. Preferably, the treated steel pipe is reduction-rolled at a cumulative reduction rate of at least about 20% at a temperature below the Ar3 transformation point.
Preferably, the composition further comprises at least one group of Group A and Group B, wherein Group A includes at least one element of about 0.1% or less of Cr, about 0.05% or less of Nb, about 0.05% or less of Ti, about 1.0% or less of Cu, about 1.0% or less of Ni, about 1.0% or less of Mo, and about 0.01% or less of B; and Group B includes at least one element of about 0.02% or less of Ca and about 0.02% or less of a rare earth metal.